In order to study the mechanism of thyroid hormones transport across the plasma membrane and intracellular distribution and localization in intact cells, fluorescent rhodamine-labeled thyroid hormones were synthesized. Rhodamine-labeled thyroxine (Rho-T4) and triidothyronine (Rho-T3) bind to the rat liver nuclear receptors with the dissociation constants of 70.8 nM and 20.3 nM, respectively. Studies with cultured GH1 cells showed patches and caps on the surface after incubation of the cells with Rho-T3. No caps and patches were observed for rhodamine-labeled thyronine. Thus the interaction of Rho-T4 and Rho-T3 is specific. The physiological significance of these phenomena is currently under study.